Explosive fracturing method

ABSTRACT

This is a method for increasing the permeability of an underground formation adjacent a well bore. The first step is to plug existing fractures in the formation from the interwell area to a location near the well bore. Next, the bore of the well and the fracture immediately adjacent to the bore are filled with a liquid explosive which is stemmed. Then the explosive is detonated, forming new fractures into the previously unfractured portion of the formation.

United States Patent Clarence R. Fast;

George C. Howard; Morton A. Mallinger, all of Tulsa, Okla.

Oct. 27, 1969 Dec. 28, 1971 Amoco Production Company Tulsa, Okla.

Inventors Appl. No, Filed Patented Assignee EXPLOSIVE FRACTURING METHOD 4 Claims, 5 Drawing Figs.

U.S. Cl 166/281, 166/299 Int. Cl E211) 43/26 Field of Search 166/281, 299, 308

[56] Reieren c es Cited UNITED STATES PATENTS 3,066,733 12/1962 Brandon 166/299 3,075,463 1/1963 Eilers et al.' 166/299 3,270,815 9/1966 Osborn et a1. 166/299 3,336,981 8/1967 Barron et a1. 166/281 3,336,982 8/1967 Woodward et al.. 166/299 2,911,046 11/1959 Yahn l66/299X Primary Examiner-Stephen J. Novosad Attorneys-Paul F. l-lawley and John D. Gassett PATENTED M02819?! 3630.279

sum 1 [IF 3 CLARENCE R. FAST GEORGE C. HOWARD MORTON A. MALLINGER INVENTORS ATTORNEY PATENTEU M028 I97! SHEET 2 [IF 3 CLARENCE R FAST GEORGE C. HOWARD MORTON A. MALLINGER INVENTORS O 3 2 3 F Qu E ,S 3 H J 3 PO W0 H mm W H N E 6 3 4 D 3 O P r 5 v 2 O 8 m m 4 4 s 6 O 5 w 6 E T DC N m N on E T AU M A so E w S C 4 6 A 4 4 4 BY ATTORNEY FIG. 3

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of increasing the permeability in the formation adjacent an oil and gas well which penetrates therethrough. It relates especially to a system in which existing fractures in a formation are first plugged and then new fractures are created by the detonation of an explosive in a well bore after the fractures are plugged.

2. Setting of the Invention Many oil and gas wells are drilled into petroleum-bearing fonnations which contain fractures, either naturally occurring or artificially induced. Many such fractures extend vertically, i.e., the fracture intercepts the well bore in more or less a vertical plane. Restimulation of such vertically fractured wells, such as by a hydraulic fracturing, usually results in extension of existing vertical fractures. In the case of a producing well, this additional stimulation does not result in desired production increases as it does not reach the unfractured matrix of the formation. In the case of injection wells, refracturing of the wells would also result in an extension of existing vertical fractures and does not produce the desired improvement in sweep efficiency of the injected fluid. On the contrary, it usually results in poor efficiency because direct water-channeling through the fractures is increased. Thus, there is a need for a technique of well stimulation to improve the productivity of producing wells and to improve sweep efficiency in injection wells.

BRIEF DESCRIPTION OF THE INVENTION Fractures which intercept the bore of an oil and gas well are first plugged. A suitable method for plugging these fractures is described in allowed copending application Ser. No. 586,484, filed Oct. 13, 1966, by Don H. Flickinger and Clarence R. Fast, and having a common assignee with the present application. A liquid explosive is placed in the well bore adjacent the producing formation. The liquid explosive is stemmed and then detonated to fracture the formation in a radial pattern about the well. Such fracturing will radiate from the original fracture surface and open up untapped areas of the reservoir to the bore.

BRIEF DESCRIPTION OF THE DRAWINGS Various objectives and a better understanding of the invention can be had from the following description taken in conjunction with the drawings.

FIG. 1 is a horizontal section view of an underground formation illustrating vertical fractures extending outwardly from the well.

FIG. 2 is similar to FIG. 1 and shows a portion of the fractures plugged.

FIG. 3 is a vertical view, partly in section, of a well penetrating the producing formation.

FIG. 4 illustrates a vertical view, partlyin section, of a well bore in which the explosive has been placed.

FIG. 5 illustrates an explosive placed in the well bore, in the unplugged portion of the fracture of the well bore and fracture of FIG. 2.

DETAILED DESCRIPTION FIG. 1 illustrates a well which has penetrated a hydrocarbon-containing formation 12. Well 10 can be either a producing well or an injection well. It is well known that an injection well is a well through which a fluid is injected to displace hydrocarbons from the formation to a second or output well. FIGS. 1, 2 and 5 are top views of a horizontal section of the formation. As illustrated, there is a fracture 14 extending from one side of the well 10 and a fracture 16 extending from the other side. Fractures l4 and 16 may be acommon fracture which has been intercepted by well bore 10. Fractures l4 and 16 are commonly known as vertical fractures, and ordinarily extend from at least the bottom to the top of a producing formation.

We want to create fractures which radiate outwardly from fractures 14 and 16. We first plug fractures 14 and 16, or at least a portion thereof. We obtain this plugging by injecting liquid-containing, finely divided, low-density, essentially nonsettling solids into said fissures through well 10 at a pressure above the fracture opening pressure but below the fracturing pressure of the formation. This can be done in accordance 1 with the teaching of said US. Pat. application Ser. No. 586,484. It is preferred that portions 14A and 16A of fractures l4 and 16 be left unplugged. As indicated in FIG. 2, sections 14A and l6A extend about 25 feet to about 50 feet from the well bore. This can be accomplished by displacing the fluid containing the finely divided solids further into the formation by a fluid such as a viscous guar gum solution.

FIG. 3 is a schematic view of well bore and auxiliary equipment for injecting the explosive into the well bore. Shown thereon, is a casing 20 set in well bore 10. Well bore 10 extends through formation ]2. However, casing 10 terminates at the top of formation 12. A tubing string 22, which may be 2% inch 0D,, is run in the well bore. This tubing string 22 contains a packer 24 which may be a type R'I'IS packer, which is commercially available from Halliburton Company, Duncan, Oklahoma. A tailpipe section 26 is run below packer 24 and has a tubing plug catcher sub 28 thereon.

We will now discuss the surface equipment shown in FIG. 3. This includes a container 30 for explosive media and a source of high-pressure nitrogen 32. A line 34 having valve 36 connects the outlet of the explosive container 30 to tubing string 22. A line 38 having valve 40 connects pump 42 to line 34 and tubing string 22. Pump 42 can be selectively connected to a sand source 44 and cement source 46 or a water source 48 through lines 50, 52 and 54, respectively, and which lines also respectively contain valves 56, 58 and 60. The annulus between casing 20 and tubing string 22 is connected to outlet line 62 to a pit now shown. Line 62 contains valve 64.

Having described the components of FIG. 3, attention will now be directed toward the operation for placing explosive in a borehole and for stemming such explosive. First, open valve 36 and close valve 40. High-pressure nitrogen is supplied through line 33 and valve 35 to container 30 to displace the liquid explosive from container 30 to tubing string 22. A suitable liquid explosive can be sensitized nitromethane which contains a thickener and other additives in proportions to meet bottom hole conditions. The formulation of the explosive will be tailored to allow ease in displacement, depending on the type'of formation and fracture system, bottom hole temperature and pressure. For example, the liquid explosive can be nitromethane thickened with nitrocellulose, containing powdered aluminum and ammonium nitrate in a stoichiometric mixture and with a suitable sensitizer such as nitric acid or an amine. A tubing wiper plug (which may be a Halliburton 5W) is first inserted in the tubing. This is followed immediately by the liquid explosive. When the desired amount of liquid explosive is injected, valve 36 is closed and valve 40 is opened. A top tubing wiper plug is released and valve 60 is opened so that water and the plug can be pumped into the tubing string 22 to displace the liquid explosive down the tubing. Valve 64 is opened as may be required to relieve fluid pressure from the annulus. As explained more fully in our copending application, it is desirable to avoid trapping air, which causes premature ignition of the explosive liquid. A sorbing solvent, such as unleaded gasoline or toluene, is inserted between plug and the explosive liquid, and between the explosive liquid and the top plug.

After the liquid explosive is displaced from tubing 22, packer 24 is unset and tubing string 22 removed from the well bore. Attention is next directed to FIG. 4 which shows a detonator 70 placed in the liquid explosive 72. Detonator 70 can be a commercially available clock-actuated time bomb. The explosive 72 extends through a major portion of formation 12 Immediately on top of explosive 72 may be a column of water 74 which may be about feet. A displacing plug can be used next to the liquid explosive if the use of water would be detrimental to the liquid explosive. A bridge plug 76 is set immediately above the water. Above the bridge plug 76 is cement 78. For most liquid explosives, about 50 to 100 feet of cement is adequate. Immediately above the cement 78 is a column of sand 80 which will ordinarily extend to the surface. Detonator 70 is set to go off ordinarily 24 to 48 hours or longer after it has been placed. This gives adequate time for stemming the well. When the explosive detonates, the explosive shatters the formation immediately around the well bore and causes fractures to extend deep into the untapped area of the reservoir rock. If the vertical fractures are plugged deeply in the formation, e.g., 100 feet to 200 feet, or more, then the detonation of the explosive liquid will tend to cause fractures to radiate from the well bore and the fracture faces in all directions. This increases the reservoir sweep efficiency tremendously.

In a slight modification of this operation, it is desired to displace the plugging material outwardly from the well bore as indicated in FIG. 2. The well is then swabbed so that fluid from fracture sections 14A and 16A is withdrawn into the well bore and removed. This permits the liquid explosive to enter fracture portions 14A and 16A, as indicated in FIG. 5. With the liquid explosive 72 in fracture portions 14A and 16A, detonation shatters the rock adjacent these fracture sections.

While the above embodiments of the invention have been described with considerable detail, it is to be understood that various modifications of the invention can be made without departing from the scope or spirit of the invention.

We claim:

1. A method of fracturing a formation penetrated by a well bore in which said formation contains fractures which comprises:

plugging the existing fractures by injecting a plugging material into such fractures,

injecting a viscous material into the well bore to drive the plugging material away from the well bore for a distance of at least about 25 feet,

injecting an explosive composition into the well bore adjacent the formation, and

thereafter detonating said explosive composition.

2. A method as defined in claim 1 including the step of stemming said explosive composition before detonation.

3. A method as defined in claim 1 including the step of injecting liquid explosive into the unplugged portion of the fracture adjacent the well bore.

4. A method of fracturing a formation penetrated by a well bore in which said formation contains fractures which comprises;

plugging the existing fractures by injecting a liquid contain ing finely divided, low-density, essentially nonsettling solids into said fractures through the well at a pressure above the fracture opening pressure but below the fracturing pressure of the formation,

injecting an explosive composition into the well bore adjacent the formation, and

thereafter detonating said explosive composition. 

2. A method as defined in claim 1 including the step of stemming said explosive composition before detonation.
 3. A method as defined in claim 1 including the step of injecting liquid explosive into the unplugged portion of the fracture adjacent the well bore.
 4. A method of fracturing a formation penetrated by a well bore in which said formation contains fractures which comprises: plugging the existing fracTures by injecting a liquid containing finely divided, low-density, essentially nonsettling solids into said fractures through the well at a pressure above the fracture opening pressure but below the fracturing pressure of the formation, injecting an explosive composition into the well bore adjacent the formation, and thereafter detonating said explosive composition. 